1. Field of the Invention
This invention relates to optical glass having high refractive index and low dispersion.
2. Description of the Prior Art
High refractive index, low dispersion optical glass heretofore has been formed of a basically B.sub.2 O.sub.3 -alkaline earth metal oxides-La.sub.2 O.sub.3 system. In an optical glass of this type, a large quantity of alkaline earth metal oxides is required. Of the alkaline earth metal oxides, the use of appreciable amounts of MgO, CaO and SrO makes the resultant glass very unstable against devitrification. Also, the use of a substantial amount of the alkaline earth metal oxide. BaO greatly diminishes the chemical durability of the resultant glass. Thus, high refractive index, low dispersion optical glass made of a system which is basically B.sub.2 O.sub.3 -alkaline earth metal oxides-La.sub.2 O.sub.3 is unsuitable for mass production because of its tendency to devitrify and its poor chemical durability, and therefore, its insufficiency as elements for optical instruments.
An improvement over such known glass is a high refractive index, low dispersion optical glass which has ZnO as one of the chief components, instead of alkaline earth metal oxides, thereby constituting an essentially three component system of B.sub.2 O.sub.3 -ZnO-La.sub.2 O.sub.3. An example of this type of glass has a composition as follows, the values being present by weight:
______________________________________ B.sub.2 O.sub.3 29-43 La.sub.2 O.sub.3 16-27.5 ZnO 17-36 SiO.sub.2 1-7 MgO + CaO + SrO 1-11 CdO 0-6.5 ______________________________________
However, such optical glass is low in La.sub.2 O.sub.3 content which serves to impart the high refractive index and low dispersion properties to the glass. Consequently, in order to maintain for this glass the high refractive index and low dispersion properties, it is necessary to increase the ZnO content which, among the chief or essential components, is next to La.sub.2 O.sub.3 in the degree of contribution for imparting high refractive index and low dispersion properties to the glass. However, this glass which contains such a substantial amount of ZnO, which is a glass network modifying oxide, has a low viscosity; also, the glass may not be said to be sufficiently stable against divitrification. This is a serious disadvantage insofar as mass-producing the glass.